It is known that the search for new glasses, particularly having a very large optical window extending to the infra-red spectrum, has led many researchers to prepare glasses which are constituted primarily of metal halides and which for a new class of the glasses, known as hologen glasses.
These halogen glasses are, during their production, contaminated, especially by OH.sup.- or O.sub.2- ions. The presence of these impurities in the halogen glasses causes the appearance of troublesome parasitic absorption bands in the vicinity of a wave length 2.9 micrometers, due to the OH.sup.- hydroxyl, and in the vicinity of 7-9 micrometers, due to the metal-oxygen bonds, see for example, M. ROBINSON et al., Mat. Res. Bull., Vol. 15, No. 6, pp 735-742 (1980). These parasitic absorptions alter the optical characteristics of the halogen glasses, and the object of the present invention is to eliminate these disadvantages.